In mass air flow sensors used in the induction passages of automotive vehicles it is desirable to obtain a signal which can be used for real time control of fuel injection. Such a signal must have a high signal to noise ratio so that the time delay associated with filtering can be nearly eliminated.
Important classes of MAF sensors comprise a small chip or substrate of semiconductor or insulator material mounted in the air stream and having surface mounted or deposited elements for sensing the air flow. These may include constant temperature anemometry or time of flight measurement. The latter class is exemplified by the U.S. Pat. No. 4,576,050 to Lambert entitled "Thermal Diffusion Fluid Flow Sensor", which is incorporated herein by reference. That patent describes a solid state sensor having a low diffusivity layer carrying a resistive heater strip and a thermoelectric detector spaced from the heater strip to detect thermal waves emitted from the heater strip. The time of transport of the thermal wave from emitter to detector is a function of the fluid flow across the substrate. U.S. Pat. No. 4,782,708 to Harrington et al entitled "Thermocouple Sensors" is related to the Lambert patent and reveals specific thermocouple materials for use as the detector.
Sensors having such extended surfaces or which are mounted on extended surfaces are subject to noise which arises from turbulence or recirculation of air, or are subject to a boundary layer of air which has a velocity lower than the average airflow in the induction passage. Either effect makes the sensor unacceptable for real time MAF signal processing. Thus the importance of correct disposition of the sensor in the passage is important.
U.S. Pat. No. 3,374,673 to Trageser entitled "Mass Flowmeter Structure" recognizes the likelihood of turbulence occurring in the flow passage and proposes to reduce such instabilities at the flow sensor by placing a foraminous structure upstream of the sensor and by incorporating a venturi restriction in the passage, the sensor being placed at the throat of the venturi as the most stable location. Both of these measures are objectionable since they introduce flow restrictions into the passage. Moreover, this patent mainly deals with turbulence and the like created upstream of the sensor and does not advise how to manage the airflow to avoid turbulence created by the sensor itself.
U.S. Pat. No. 4,317,385 to Lauterbach also imposes restrictions in the flow passage in an attempt to create a region of stabilized flow for flow measurement. A nozzle-like restriction is formed in the passage and the sensor element is mounted on the wall of the restriction or on a wedge shaped or lenticular element suspended on the center axis of the restriction, upstream of the throat of the restriction. Again, it is desirable to avoid such a restriction in the air flow passage.
Generally, the flow sensors are unidirectional; that is, they measure the airflow in only one direction or they measure the airflow in either direction without discrimination so that in the case of pulsating airflow a reverse airflow is added to forward airflow to give a false measure. This is usually the case with constant temperature anemometers: the cooling effect of the air on the airflow sensor is independent of the flow direction so that reverse flow is added to forward flow. In some cases the flow passage geometry is designed to prevent reverse flow from affecting the airflow sensor, so that the reverse flow is ignored in the flow measurements. Reverse flow does occur in the induction passages of internal combustion engines due to pulsation caused by the engine valve action and is especially noticeable in engines with a small number of cylinders. To get an accurate measure of the net airflow to such an engine it is necessary to separately measure the forward and reverse airflows and calculate the difference.